1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device. More specifically, the invention relates to a one-time programmable (OTP) memory to which information can be written only once.
2. Description of the Related Art
Recently, an OTP memory using storage elements of an insulating film breakdown type, such as an electrical-fuse element (e-fuse element) and an anti-fuse element, has been proposed. The OTP memory stores information by applying a high voltage that exceeds the maximum rating to a semiconductor element having a metal oxide semiconductor (MOS) structure and breaking a gate insulating film of the semiconductor element. The OTP memory using the e-fuse element is used to store information “0” in the e-fuse element whose insulating film has not been broken and store information “1” in the e-fuse element whose insulating film has been broken. The OTP memory is also used to store information for replacing a defective element with a dynamic random access memory (DRAM) or the like.
Conventionally, an OTP memory using an optical-fuse element whose storage node is broken by laser beams has been often used. The e-fuse element has originally been used as an alternative to the optical-fuse element. The following example has been proposed: one storage cell is configured for each e-fuse element to include a sense circuit for reading information and a latch circuit for latching the information. See, for example, H. I to et al., “Pure CMOS One-Time Programmable Memory using Gate-Ox Anti-Fuse,” Proceedings of the IEEE 2004 Custom Integrated Circuits Conference, pp. 469-472.
With the above configuration, however, each e-fuse element (or storage cell) includes a sense circuit and a latch circuit. This configuration has the advantage that the stably operated OTP memory can easily be designed, whereas it has the problem that the OTP memory greatly increases in size.
Particularly when a write control circuit is provided for each e-fuse element to control a series of write operations (program operations) such as precharge and application of stress, the OTP memory cannot be decreased in size though an operation of writing information to each e-fuse element can exactly be performed to achieve, for example, a complicated sequence.